The production of reformulated gasoline satisfying new environmental regulations requires, in particular, a reduction in the concentration of olefins and/or aromatics (especially benzene), also sulphur (including mercaptans).
Catalytic cracking gasolines have high olefin contents, and the sulphur present in the gasoline pool is about 90% attributable to FCC gasoline.
Hydrotreatment of the feed sent for catalytic cracking can result in gasolines which typically contain 100 ppm of sulphur. Units for hydrotreating FCC feeds operate, however, under severe temperature and pressure conditions, which necessitates high investment.
Hydrotreatment of catalytic cracking gasolines can reduce both the sulphur content and the olefin content in the cut. However, this has the major disadvantage of causing a very large barrel octane drop in the cut, because of saturation of the olefins.
FCC gasoline hydrotreating processes have already been proposed. As an example, U.S. Pat. No. 5,290,427 describes a process consisting of fractionating the gasoline, desulphurizing the fractions and converting the gasoline fraction over a ZSM-5 zeolite.
U.S. Pat. No. 5,318,690 proposes a process including fractionation of the gasoline, sweetening the light fraction, hydrodesulphurizing the heavy fraction, then converting it over ZSM-5 and re-desulphurizing under mild conditions. That technique is based on separating the raw gasoline to obtain a light fraction which is practically free of sulphur-containing compounds other than mercaptans, so that that fraction can be treated by sweetening alone to remove the mercaptans. In this fashion, the heavy fraction contains a relatively large quantity of olefins which are partially saturated during hydrotreatment. In order to prevent this octane number drop, that patent recommends cracking over ZSM-5 to produce olefins, but this is to the detriment of the yield. Further, the olefins can be reconstituted in the presence of H2S to form mercaptans, which has the disadvantage of requiring additional sweetening or a desulphurizing step.
In a further prior art method used by the refiner to treat the sulphur problem in gasolines, the fraction with a boiling point of at least 180° C., which contains most of the sulphur-containing compounds other than mercaptans, is separated. This fraction is then downrated with LCO (light cycle oil) and is generally not upgraded, or it is used as a feed diluent
We have developed a process for the production of gasolines with a low sulphur content from catalytic cracking, which can upgrade the whole of the gasoline cut, and reduce the sulphur content of the gasoline cut to very low levels, without dropping the gasoline yield, and minimise the octane drop.
More precisely in the process of the invention, the raw gasoline is fractionated into at least one light cut with a boiling point of 210° C. or less containing the major portion of the olefins and mercaptans, and at least one heavy fraction. The light cut undergoes mild hydrotreatment in the presence of hydrogen with a catalyst containing at least one group VIII metal and/or at least one group VI metal, at a temperature of 160-380° C., at a pressure of 5-50 bar, and the effluent obtained is stripped to eliminate H2S. The light fraction undergoes sweetening which is carried out using at least one of the following methods:                before the mild hydrotreatment step, treating the light cut in the presence of hydrogen using a catalyst containing 0.1-1% of palladium deposited on a support, at a temperature of 50-250° C., at a pressure of 4-50 bar;        extractive sweetening of the effluent obtained after mild hydrotreatment and stripping;        sweetening the effluent obtained after mild hydrotreatment and stripping, using an oxidizing agent, a catalyst and an alkaline base which may or may not be incorporated into the catalyst.        
The feed is a catalytic cracking gasoline, in which the boiling point range typically extends from C5 to 220° C. The end point of the gasoline cut depends, of course, on the refinery and on market requirements, but are generally within the limits indicated above.
The sulphur content of these gasoline cuts produced by catalytic cracking (FCC) depends on the sulphur content of the feed which undergoes FCC, also the end point of the cut. Light fractions naturally have a lower sulphur content than the heavier fractions. In general the sulphur content of the whole of the FCC gasoline cut is over 100 ppm by weight and usually over 500 ppm by weight. For gasolines with end points of more than 200° C., the sulphur contents are often over 1000 ppm by weight, and in some cases can reach values of the order of 4000 to 5000 ppm by weight.
In accordance with the invention, the raw gasoline from catalytic cracking is fractionated into at least one light cut and at least one heavy cut.
The light cut has an end point of 210° C. or less, advantageously 180° C. or less, preferably 160° C. or less and more preferably 145° C. or less.
The light fraction of the gasoline cut contains relatively few sulphur-containing compounds, the majority of which are present in the form of mercaptans, while the sulphur-containing compounds in the heavier fractions are present in the form of substituted or unsubstituted thiophenes, or heterocyclic compounds such as benzothiophene which, in contrast to mercaptans, cannot be eliminated by extractive processes. These sulphur-containing compounds are consequently eliminated by hydrotreatment. The light fraction is relatively rich in olefins, and the sulphur is essentially present in the form of mercaptans, while the heaviest cut is relatively depleted in olefins and is characterized by much higher sulphur contents.
More generally, and in contrast to the prior art, the cut point is selected so as to maximise the olefin content in the light cut.
The catalytic cracking (FCC) gasoline cut is thus fractionated into at least two fractions, which then undergo different desulphurization treatments. The light fraction undergoes a desulphurization treatment constituted by mild hydrogenation, optionally preceded by selective hydrogenation of the diolefins. The hydrogenation conditions are selected so as to be mild to minimise saturation of high octane number olefins. Desulphurization is thus not complete but it can eliminate practically all of the sulphur-containing compounds other than the mercaptans so that essentially mercaptans remain in the cut. They are then eliminated by sweetening. This sweetening step can be extractive sweetening or sweetening by fixed bed catalytic oxidation of the mercaptans.